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EOS 170 > Lecture 9 - Introduction to Volcanoes > Flashcards

Lecture 9 - Introduction to Volcanoes Flashcards

1
Q

What is a volcano?

A

an opening in Earth’s crust through which molten rock, hot rock fragments, and gases escape in a volcanic eruption

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2
Q

What is lava?

A

molten rock at the Earth’s surface, erupted from a volcano

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3
Q

What are some examples of different forms that lava can take?

A

bombs
flows
ash fall

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4
Q

What is magma?

A

molten rock situated within the Earth, within a magma chamber or conduit

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5
Q

How does a volcanic eruption start?

A

An eruption starts with magma at depth. Gases (H2O, CO2, etc.) are kept dissolved in solution by high pressure.

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6
Q

What happens in an eruption as magma rises?

A

As magma rises, pressure is reduced. Gases come out of solution (exsolution) to form bubbles, and decompression melting may occur. Surrounding bedrock can also melt and mix with the magma.

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7
Q

During an eruption what propels magma upwards?

A

The gas bubbles propel the magma upwards. Bubble volume may overwhelm the magma, fragmenting it into pieces and exploding out as a gas jet.

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8
Q

How does the plume form?

A

The gas jet draws in surrounding air, forming a buoyant plume.

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9
Q

What happens when lava or magma cools and solidifies?

A

atoms arrange themselves into crystals

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10
Q

What are volcanic rocks formed from?

A

They are formed from lava that solidified above the surface (“extrusive”).
They are much more fine grained.

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11
Q

What are plutonic rocks formed from?

A

They are formed from magma that solidified below the surface (“intrusive”).
They are much more coarse grained.

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12
Q

What are the characteristics of basalt?

A

~50% SiO₂
Low H₂O content
Fine grained (volcanic)
Oceanic

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13
Q

What are the characteristics of andesite?

A

Between ~50% SiO₂ and ~70% SiO₂
Between low and high H₂O content
Fine grained (volcanic)
Between oceanic and continental

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14
Q

What are the characteristics of rhyolite?

A

~70% SiO₂
High H₂O content
Fine grained (volcanic)
Continental

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15
Q

What are the characteristics of gabbro?

A

~50% SiO₂
Low H₂O content
Coarse grained (plutonic)
Oceanic

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16
Q

What are the characteristics of diorite?

A

Between ~50% SiO₂ and ~70% SiO₂
Between low and high H₂O content
Coarse grained (plutonic)
Between oceanic and continental

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17
Q

What are the characteristics of granite?

A

~70% SiO₂
High H₂O content
Coarse grained (plutonic)
Continental

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18
Q

What is viscosity?

A

It is a measure of internal resistance to flow. Low viscosity materials flow easily (e.g. water), and high viscosity materials flow slowly (e.g. toothpaste).

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19
Q

What does magma/lava viscosity depend on?

A

Higher temperature = lower viscosity
Higher crystal content = higher viscosity
Higher silica content = higher viscosity

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20
Q

Where (and how) does new oceanic crust form?

A

New oceanic crust forms at mid-ocean ridges by decompression melting of asthenosphere.

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21
Q

What is the relative viscosity of magma at mid ocean ridges?

A

Magma is high temperature, low H₂O content, low SiO₂ content, and therefore low viscosity.

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22
Q

What happens when basalt erupts underwater?

A

Contact with cold water quenches the surface and the lava forms a distinctive pillow shape.

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23
Q

What is an Icelandic type eruption? Why do they occur?

A

Iceland sits on a mid-ocean ridge that is above sea level. Because of the low viscosity, gas bubbles can escape and eruptions are peaceful.

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24
Q

What is a landform associated with Icelandic type eruptions?

A

The low-viscosity basalt spreads out into a horizontal layer. Over time, these stack up to form flood basalts.

25
Q

What is the relative viscosity of magma at oceanic hotspots?

A

Magma at oceanic hot spots is high temperature, low in H₂O and SiO₂, and thus low viscosity. Gas bubbles can escape and eruptions are peaceful.

26
Q

What is a Hawaiian type eruption?

A

In Hawaiian-type eruptions, basaltic lava spills out of fissures as lava fountains and flows downhill as lava rivers.

27
Q

What is the viscosity of magma in Hawaii compared to magma in Iceland?

A

Magma erupted in Hawai’i is slightly more viscous than that in Iceland.

28
Q

What is a landform associated with Hawaiian type eruptions?

A

The giant shield volcano, Mauna Loa. This has an elevation of ~9 km above the Pacific sea-floor (~4 km above sea-level) but is ~200 km wide. By volume, it is the largest sub-aerial volcano on Earth!

29
Q

What happens to the solidus at volcanic arcs?

A

At volcanic arcs, addition of water to the mantle from the cold slab lowers the solidus, which touches the geotherm.

30
Q

What is a characteristic of basaltic magma at volcanic arcs?

A

Lower temperature.
Higher H₂O content.
To erupt, the magma must pass through ~40 km of arc crust.
Melting and mixing increases magma SiO₂ content, raising viscosity further.

31
Q

How are basaltic, andesitic and rhyolitic magmas formed?

A

H₂O is released from slab, melting overlying asthenosphere, producing basaltic magma. Heat melts continental crust. This mixes with basalt to produce andesitic magma. Further melting and mixing of continental crust produces rhyolitic magma.

32
Q

What are the three types of eruption characteristic of volcanic arcs called?

A

Strombolian-type eruptions
Vulcanian-type eruptions
Plinian-type eruptions

33
Q

What types of magma are associated with Strombolian-type eruptions?

A

Medium viscosity basaltic to andesitic magmas.

34
Q

How explosive are Strombolian-type eruptions? Why?

A

They are mildly-explosive. This is due to the magma trapping gas bubbles.

35
Q

What is a landform associated with Strombolian-type eruptions?

A

Scoria cones which are mounds of volcanic rock fragments piled up next to a volcanic vent. The summit crater may hold a lava lake during an eruption.

36
Q

Where can you find some examples of scoria cones?

A

BC has many scoria cones in the Stikine Volcanic Belt (e.g. Tseax) and the Wells Gray–Clearwater Volcanic Field (e.g. Kostal).

37
Q

What types of magma are associated with Vulcanian-type eruptions?

A

Medium-to-high viscosity magmas, typically andesitic in composition.

38
Q

How explosive are Vulcanian-type eruptions? Why?

A

They are explosive as the magma is sufficiently viscous. Eruptions send rock and ash into the atmosphere, forming a plume.

39
Q

What is a landform associated with Vulcanian-type eruptions?

A

Vulcanian-type eruptions produce stratovolcanoes: tall, steep-sided, symmetrical peaks built of alternating ash and viscous magma flows.

40
Q

What types of magma are associated with Plinian-type eruptions?

A

High viscosity rhyolitic magmas

41
Q

How explosive are Plinian-type eruptions? Why?

A

High viscosity rhyolitic magmas trap gas bubbles most effectively, producing violent eruptions (e.g. Mt St. Helens, 1980).

42
Q

What is a landform associated with Plinian-type eruptions?

A

They are also associated with stratovolcanoes, but in these cases emptying of the magma chamber can lead to collapse of the volcanic cone, forming a caldera.

43
Q

What is a historical example of a Plinian-type eruption?

A

Pliny the Younger described Mt Vesuvius erupting in 79 CE. The Roman city of Pompeii was buried in a pyroclastic flow.

44
Q

What is an ultra-Plinian-type eruption?

A

A sixth class of volcanic eruption, Ultra-Plinian, is needed to account for the largest eruptions of all, defined by >25 km high plumes and >10 km³ volumes.

45
Q

What is an example of an ultra-Plinian-type eruption?

A

~630 ka Yellowstone (WY): caldera 75 x 45 km, volume ~1000 km³.
The Yellowstone supervolcano occupies a continental hot spot. Magma is rhyolitic since it must rise through thick continental crust.

46
Q

What is the Volcanic Explosivity Index (VEI)?

A

It is another way of classifying volcanic eruptions. It is calculated on the basis of three factors:
the volume of material erupted
the height of the eruption column
the duration of the major eruptive blast

47
Q

What is volcanic glass and how is it produced?

A

Lava coming into contact with cool air or water can be quenched, solidifying so quickly that crystallization cannot happen. This produces glass, such as obsidian.

48
Q

What is pumice?

A

It is a frothy volcanic glass. It is produced when gas escapes quickly from lava. By volume, it can comprise ~90% holes!

49
Q

What are pyroclasts (tephra) ?

A

Chunks of magma and surrounding rock which are blasted out by explosive eruptions (pyro = fire, clastic = fragments of rock).

50
Q

In what ways can pyroclasts fall from the volcanic plume?

A

As ash fall, or entrained in a denser gravity current known as a pyroclastic flow.

51
Q

How are particles deposited differently by ash fall and pyroclastic flows?

A

In ash fall, pyroclasts are sorted into layers of different grain sizes, forming a gradient with distance from the volcano: coarser particles fall closest to it, finer particles are transported greater distances.
Pyroclastic flow deposits are more chaotic, with little or no sorting of the various sizes of particles.

52
Q

What are the main elements in volcanic gases?

A 
hydrogen
oxygen
carbon
sulphur
chlorine
nitrogen
These form the molecules water vapour (>90%), CO₂, CO, CH₄, N₂, SO₂, H₂S, H₂ and HCl.
53
Q

What molecules do volcanic rocks contain?

A

Volcanic rocks are rich in Mg and Fe, derived from the asthenosphere. On the way up to the surface, the magma becomes enriched in elements abundant in the continental crust: Al, Ca, K, and Na.

54
Q

Give a brief summary of the relative viscosity, volatility and volume of Icelandic-type eruptions.

A

Composition: Basalt
Viscosity: Low
Volatility: Low
Volume: Very large

55
Q

Give a brief summary of the relative viscosity, volatility and volume of Hawaiian-type eruptions.

A

Composition: Basalt
Viscosity: Low
Volatility: Low
Volume: Large

56
Q

Give a brief summary of the relative viscosity, volatility and volume of Strombolian-type eruptions.

A

Composition: Basalt/Andesite
Viscosity: Low/Medium
Volatility: Medium
Volume: Small

57
Q

Give a brief summary of the relative viscosity, volatility and volume of Vulcanian-type eruptions.

A

Composition: Basalt → Rhyolite
Viscosity: Medium/High
Volatility: Medium/High
Volume: Large

58
Q

Give a brief summary of the relative viscosity, volatility and volume of Plinian-type eruptions.

A

Composition: Andesite/Rhyolite
Viscosity: High
Volatility: High
Volume: Very large

EOS 170 (22 decks)

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